1. Field of the Invention
The present invention relates to a toroidal continuously variable transmission, and particularly to a half toroidal continuously variable transmission, that can be used as an automatic transmission for an automobile or as a transmission for all kinds of industrial machinery.
2. Description of the Related Art
The use of a toroidal continuously variable transmission as an automobile transmission, for example, is described in many publications, and the toroidal continuously variable transmission has been partially implemented and it has been well known. Particularly, toroidal continuously variable transmissions having various construction are disclosed in Japanese Patent Application Publication No. 2008-25821. FIG. 26 illustrates the basic construction of a currently implemented toroidal continuously variable transmission as disclosed in this document.
First, a first example of this conventional construction is explained. A pair of input side disks 1a, 1b are supported around an input rotating shaft 2 such that, when the inner surfaces 3 on the input side thereof, which are toroidal curved surfaces (concave surface having an arc shaped cross section), face each other, the disks 1a, 1b are concentric with each other and freely rotate in synchronization.
An output cylinder 5 having an output gear 4 fastened around the outer peripheral surface in the midsection thereof is supported around the middle section of the input rotating shaft 2 such that it rotates freely with respect to the input rotating shaft 2. In addition, output side disks 6 are supported around the both end sections of this output cylinder 5 via spline joints such that they freely rotate in synchronization with the output cylinder 5. In this state, inside surfaces 7 on the output side of both output side disks 6, which are toroidal curved surfaces, face the inside surfaces 3 on the both input sides.
Two power rollers 8, the peripheral surfaces thereof being spherical convex surfaces, are placed around the input rotating shaft 2 in each of the portions (cavities) between the inner surfaces 3, 7 on the input sides and output sides. These power rollers 8 are supported by the inside surface of the respective trunnions 9 by way of support shafts 10, whose base side section and tip side section are eccentric, and a plurality of rolling bearings such that they can freely rotate around the tip side section of the support shafts 10, and can pivot and displace a little around the base side section of the support shafts 10.
Each trunnion 9 can freely pivot and displace around tilt shafts that are concentrically provided on both ends in the lengthwise direction (front to rear direction of FIG. 26) of each trunnion 9. The operation that pivots (tilts) each trunnion 9 is performed by a hydraulic actuator displacing the trunnion 9 in the axial direction of the tilt shaft. When changing speed, each trunnion 9 is displaced in the axial direction of the tilt shaft by supplying hydraulic fluid to or removing hydraulic fluid from each actuator. As a result, the direction in which the force that acts in the tangential direction of the contact section (traction section) between the peripheral surface of each power roller 8 and the inside surfaces 3, 7 on the input side and output side changes (side slipping occurs), so each trunnion pivots and displaces around each tilt shaft.
During operation of a toroidal continuously variable transmission as described above, one of the input-side disks la (left disk in FIG. 26) is rotated and driven by the drive shaft 11 via a loading cam pressure unit 12. As a result, the pair of input-side disks 1a, 1b that are supported on both ends of the input rotating shaft 2 rotate in synchronization while being pressed in a direction toward each other. Moreover, this rotation is transmitted to both output-side disks 6 via each power roller 8 and output from the output gear 4.
In the case of changing the rotational speed ratio between the input rotating shaft 2 and the output gear 4, first, when reducing the speed between the input rotating shaft 2 and the output gear 4, each trunnion 9 is pivoted to the position illustrated in FIG. 26, to brace the peripheral surface of each power roller 8 in contact with both the section toward the center of the inside surface 3 on the input side of each input-side disk 1a, 1b and the section toward the outer periphery of the output side inside surface 7 of both output-side disks 6. On the other hand, when increasing the speed, each trunnion 9 is pivoted to the opposite direction of that illustrated in FIG. 26, to brace the surface around each power roller 8 into contact with both the section toward the outer periphery of the inside surface 3 of the input side of both input-side disks 1a, 1b, and the section toward the center of the inside surface 7 of the output side of both output-side disks 6. By setting the pivot angle of each trunnion 9 to an intermediate angle, an intermediate speed ratio (transmission gear ratio) is obtained between the input rotating shaft 2 and the output gear 4.
When operating the toroidal continuously variable transmission as described above, each member to which the power is transmitted, or in other words, each input-side and output-side disk 1a, 1b, 6 and each power roller 8 elastically deforms based on the pressure force (thrust force) that is generated by the pressure unit 12. In addition, with this elastic deformation, each disk 1a, 1b, 6 displaces in the axial direction. Moreover, the pressure force that the pressure unit 12 generates becomes larger the larger the torque that is transmitted by the toroidal continuously variable transmission becomes, and as that happens the amount of elastic deformation of each member becomes large. Therefore, in order to properly maintain the contact state between each inside surface 3, 7 on the input side and output side and the peripheral surface of each power roller 8, regardless of fluctuation in the torque, a mechanism that causes each power roller 8 to displace in the axial direction of each disk 1a, 1b, 6 toward each trunnion 9 is necessary. In the case of the first example of conventional construction illustrated in FIG. 26, each power roller 8 is caused to displace in the axial direction by pivoting and displacing the tip side section of each support shaft 10 that supports the power roller 8 around the base side section of the support shaft 10.
In the case of the first example of this kind of conventional construction, construction for causing each power roller 8 to displace in the axial direction is complex, making the manufacturing, management and assembly work of parts troublesome and an increase in cost is unavoidable. Construction as illustrated in FIG. 27 to FIG. 33 is disclosed in Japanese Patent Application Publication No. 2008-25821 as technology for solving such a problem.
In the following, a second example of conventional construction as illustrated in FIG. 27 to FIG. 33 is explained. A feature of this second example of conventional construction is the construction of a section that supports a power roller 8a with respect to a trunnion 9 such that it can displace in the axial direction of the each input-side and output-side disk 1a, 1b, 6 (see FIG. 26), where the construction and operation of the overall toroidal continuously variable transmission is the same as in the first example of conventional construction illustrated in FIG. 26.
The trunnion 9a of this second example of conventional construction comprises a pair of tilt shafts 13 on both ends of the trunnion 9 that are concentric with each other, and a support beam 15 of which at least the side surface on the inside (upper side in FIGS. 28, 30 to 32) in the radial direction of both the input-side and output-side disks 1a, 1b, 6 is a cylindrical convex surface 14. Both tilt shafts 13 are pivotally supported by a yoke (this construction is known, so is not illustrated in the figures) or the support plate section of pivot frame by way of radial needle bearings 16 (see reference number 13 and 14 in FIGS. 71 and 72 of Japanese Patent Application Publication No. 2008-25821).
As illustrated in FIG. 28 and FIG. 31, the center axis A of the cylindrical convex surface 14 is parallel with the center axis B of both tilt shafts 13, and is located further outside in the radial direction of the disks 1a, 1b, 6 than the center axis B of both tilt shafts 13. Moreover, a partially cylindrical concave section 19 is provided on the outside surface of the outer race 18 of a thrust ball bearing 17 that is provided between the support beam 15 and the outside surface of the power roller 8 such that it passes over this outside surface in the radial direction. In addition, this concave section 19 fits with the cylindrical convex surface 14 of the support beam 15 and supports the outer race 18 such that it can pivot and displace in the axial direction of each disk 1a 1b, 6 with respect to the trunnion 9a. The radius of curvature r19 of the cross-sectional shape of the concave section 19 equal to or greater than the radius of curvature r14 of the cross-sectional shape of the cylindrical convex surface 14, and this concave section 19 and cylindrical convex surface 14 come in direct contact over the entire surface or in the portion near the bottom section of this concave section 19.
The support shaft 10a is fastened to and integrated with the outer race 18 at a central section on the inside surface of the outer race 18, and the power roller 8a is supported around this support shaft 10a by way of a radial needle bearing 20 such that it can rotate freely. Moreover, a downstream side oil supply path 21 for supplying lubrication oil to the thrust ball bearing 17 and radial needle bearing 20 is provided in the inside of the outer race 18 and support shaft 10a, and an upstream side oil supply path 22 that connects to this downstream side oil supply path 21 is provided in the inside of the support beam 15. Both of these oil supply paths 21, 22 are connected to each other by way of a concave hole 31 that is formed in the central section of the concave section 19 regardless of the pivot displacement of the outer race 18, and furthermore, an oil supply pipe 23 that connects to the upstream side oil supply path 22 is provided outside the support beam 15. The end section on the upstream side of this oil supply pipe 23 opens into the inner radial side of a pulley 24 that is provided on the end section of the trunnion 9 for suspending a synchronizing cable, and is able to supply lubrication oil through the inner radial side of this pulley 24.
Furthermore, a pair of stepped surfaces 25 that face each other are provided on the inside surface of the trunnion 9a where both end sections of the support beam 15 connect with pair of tilt shafts 13. Moreover, both of these stepped sections 25 and the outer surface of the outer race 18 of the thrust ball bearing 17 come in contact or come close to each other, so that the traction force that is applied to the outer race 18 from the power roller 8a can be supported by either of the stepped surfaces 25.
With the toroidal continuously variable transmission of this second example of conventional construction described above, construction wherein the power roller 8a is displaced in the axial direction of each disk 1a, 1b, 6 such that contact between the surface around this power roller 8a and each disk 1a, 1b, 6 is properly maintained regardless of change in the amount of elastic deformation of each component member, can be made easily and at low cost.
In other words, in the case of operating the toroidal continuously variable transmission, if there is a necessity to displace each power roller 8a in the axial direction of each disk 1a, 1b, 6 based on the elastic deformation of the each of the input-side and output-side disks 1a, 1b, 6 and each power roller 8a, the outer race 18 of the thrust ball bearing 17 that supports each power roller 8a such that it freely rotates, pivots and displaces around the center axis A of the cylindrical convex surface 14 while at the same time the outer race 18 causes a contact surface between the partial cylindrical shaped concave section 19 that is provided on the outside surface of the outer race 18 and the cylindrical convex surface 14 of the support beams to slide. Based on this pivotal displacement, the portion of the peripheral surface of each power roller 8a that comes in rolling contact with the one side surface in the axial direction of each disk 1a, 1b, 6 displaces in the axial direction of each disk 1a, 1b, 6 and the contact state above is properly maintained.
As describe above, the center axis A of the cylindrical convex surface 14 is located further on the outside in the radial direction of each disk 1a, 1b, 6 than the center axis B of the tilt shafts 13, which become the center of pivoting of each trunnion 9a when changing speed. Therefore, the pivot radius of pivotal displacement around the center axis A of the cylindrical convex surface 14 is greater than the pivot radius during the speed change operation, and the effect on fluctuation of the transmission ratio between the input-side disks 1a, 1b and the output side disks 6 decreases to within a range that can be ignored or easily corrected.
As illustrated in FIG. 32A, in the case of embodying the second example of conventional construction described above, the radius of curvature r19 of the cross-sectional shape of the concave section 19 is typically larger than the radius of curvature r14 of the cross-sectional shape of the cylindrical convex surface 14 (r19>r14). The reason for this is that when both radii of curvature r19, r14 are equal to each other (r19=r14) the inner surface of the concave section 19 and the cylindrical convex surface 14 come in contact over the enter area of the portions that face each other, making it difficult to feed lubrication oil to this area of contact. The inner surface of the concave section 19 and the cylindrical convex surface 14, when in a state of being in contact by a large surface pressure, move and displace with respect to each other, so when the feeding of lubrication oil to this area of contact is poor, it becomes easy for large friction due to metallic contact to occur.
However, even when the radius of curvature r19 of the cross-sectional shape of the concave section 19 is greater than the radius of curvature r14 of the cross-sectional shape of the cylindrical convex surface 14 (r19>r14), the following problems arise. In other words:
(1) It becomes easy for friction to occur locally between the inner surface of the concave section 19 and the cylindrical convex surface 14.
(2) It is difficult to maintain durability of the thrust ball bearing 17.
The reason that problem (1) occurs is that, as illustrated in FIG. 32(A), the inner surface of the concave section 19 and the cylindrical convex surface 14 are in a state of nearly tangential contact only at or near the bottom section of the concave section 19, and the surface pressure at the area of contact becomes high.
The reason that problem (2) occurs is that, due to the large thrust load that is applied to the thrust ball bearing 17 from the power roller 8a, the outer race 18 elastically deforms such that the inside surface side thereof forms a partial convex cylinder shape. As is known in the technical field of toroidal continuously variable transmissions, when operating a half toroidal type toroidal continuously variable transmission that is the object of the present invention, a large thrust load is applied to the thrust ball bearing 17 from the power roller 8a. In addition, as exaggeratedly illustrated in FIG. 32B, due to this thrust load, the output race 18 of the thrust ball bearing 17 elastically deforms such that the inner surface of the concave section 19 follows the cylindrical convex surface 14, or in other words, such that the both surfaces come into contact over nearly the entire surface thereof. Moreover, due to this elastic deformation, the distance between the outer race track 28 that is provided on the inside surface of the outer race 18 and the inner race track 27 that is provided on the outside surface of the power roller 8a becomes uneven in the circumferential direction of the outer race 18. Furthermore, due to the difference in the distance between both of these tracks 28, 27, the surface pressure of the area of rolling contact between these tracks 28, 27 and the rolling surface of the balls 26 (see FIG. 33) becomes very uneven in the circumferential direction of the outer race 18. More specifically, as indicated by a in FIG. 33, the surface pressure at the area of rolling contact between both tracks 28, 27 and the rolling surfaces of the balls 26 becomes excessive at two locations on opposite sides in the length direction (axial direction of the support beam 15) of the trunnion 9a. As a result, the rolling fatigue life of these tracks 28, 27 becomes very short at these two locations on opposite sides in the length direction, and becomes an obstacle in maintaining durability of the toroidal continuously variable transmission.
In addition, in a toroidal continuously variable transmission such as the first example of conventional construction, both a normal force (force that occurs due to the pressure force necessary for traction drive) that is perpendicular to the contact point between the power roller 8 of the toroidal continuously variable transmission and disks 1a, 1b, 6, and a tangential force (traction force) that is parallel to the direction of rotation of the power roller 8 act on the power roller 8 at the point of contact. The trunnion 9 supports these two forces by way of the power roller 8. For example, the normal force is transmitted to the trunnion 9 by way of a thrust needle bearing 32 that is located between the outside surface of the outer race 18 of the thrust ball bearing 17, of which the power roller 8 functions as the inner race, and the inside surface of the support beam 15 of the trunnion 9.
A circular hole in which the base side section of a support shaft 10 as described above is inserted is provided in the trunnion 9, and the radial needle bearing that supports the base side section of the support shaft 10 such that it rotates freely is provided inside that circular hole such that the tangential force above is transmitted to the trunnion 9 from the support shaft 10 via the radial needle bearing. Therefore, it is necessary to provide a circular hole (through hole or partial hole) in the trunnion 9 in this way that will receive the support shaft 10 and radial needle bearing.
Therefore, by forming a circular hole in the support beam section 15 of the trunnion 9, the rigidity of the trunnion 9 is decreased, which becomes a problem, in that deformation due to the normal force from the power roller 8 becomes large, or stress increases, for example.
When deformation of the trunnion 9 becomes large, there is also a possibility that this deformation will cause problems such as cause the position of the contact point between the disks 1a, 1b, 6 and the power roller 8 to shift from the design value and cause the surface pressure to increase at that contact point, or that will cause the transmission ratio to shift. In addition, when controlling the transmission by using a precession cam, the position of the precession cam shifts due to deformation of the trunnion 9, and this also becomes a major cause of shifting of the transmission ratio.
In order to prevent such problems, and to prevent damage to the trunnion 9 caused by an increase in stress as described above, and as the thickness of the parts of the trunnion 9 is increased in order to maintain sufficient rigidity of the trunnion 9 in which a circular hole is formed, the weight of the trunnion 9 increases, and thus the overall weight of the transmission increases, causing other problems to occur such as a drop in fuel efficiency or increase in cost.
To solve this problem, construction is disclosed in which the tangential force can be supported without forming a circular hole in the trunnion 9. For example, in a third example of conventional construction as disclosed in Japanese Patent Application Publication No. 2001-12574, a roller bearing is located between the outer peripheral surface of the outer race 18 and the portion of the trunnion 9 that faces the outer peripheral surface of the outer race 18 such that the tangential force is transmitted from the power roller 8 to the trunnion 9 by the way of that roller bearing, and that tangential force is supported by the trunnion 9.
In this way, it is not necessary to support the base side section of the support shaft 10 by a radial needle bearing that is provided in a circular hole in the trunnion 9, and from a structural aspect, the circular hole and radial needle bearing in the trunnion 9 are eliminated, and there is also no need for the base side section of the support shaft 10, so there only needs be a shaft section that is integrated with the outer race, which corresponds to the tip side section of the support shaft 10. However, in this case, it is necessary that a track surface for the roller bearing be formed around the outer peripheral surface of the outer race 18, so there is a possibility that the processing cost will increase.
On the other hand, in the second example of conventional construction disclosed in Japanese Patent Application Publication No. 2008-25821, construction is such that the outer peripheral surface of the outer race 18 comes in direct contact with the portion of the trunnion 9a that faces that outer peripheral surface, and the tangential force described above is transmitted from this area of contact. In this case, it is not necessary to provide the roller bearing above, so there is no need to provide a track surface around the outer peripheral surface of the outer race 18, and thus it is possible to reduce the cost.
However, a track groove (ball groove) for the rolling bodies (balls) of the thrust ball bearing 17 are formed in the outer race 18, where the outer peripheral surface thereof comes in direct contact or indirect contact by way of a member such as a roller bearing with the trunnion 9, 9a, along the circumferential direction on the side near the outer peripheral surface of the inside surface of the outer race 18, so the rigidity of the portion of the inside surface side of the thickness from the outside surface to the inside surface of the portion of the outer perimeter of the outer race 18 that becomes the power roller 8, 8a side becomes low.
In other words, in the case of construction wherein of the range of thickness from the inside surface to the outside surface of the outer race 18, in the range of the depth of the track groove on the inside surface, there is only sufficient thickness in the radial direction from the outer peripheral surface of the outer race 18 to the track groove, and tangential force is transmitted by bringing the outer peripheral surface of the outer race 18 in contact with the trunnion 9, 9a, so the stress at the portion on the inside surface side of this outer peripheral portion of the outer race 18, which is a corner section between the outer peripheral surface and inside surface of the outer race, increases.
Moreover, as described above, even though the rigidity of the trunnion 9, 9a is high, there is a possibility that the trunnion 9, 9a will deform a little, and in this case, the force acts from the trunnion 9, 9a toward the outer surface of the outer race 18, and stress at the portion on the inside surface side of this outer portion of the outer race 18 further increases.
Furthermore, even if the rigidity of the trunnion 9, 9a becomes high, there is a possibility that the trunnion 9, 9a will be slightly deformed, and, in this case, the force acts from the trunnion 9, 9a to the outer peripheral surface of the outer race 8 and the stress increases at the portion on the inside surface side of this outer peripheral portion of the outer race 18.
Therefore, in order that the stress that occurs as described above becomes equal to or less than a reference value or within an allowable range, increasing the thickness of the outer race 18 is feasible, however, in this case, problems occur such as the weight of the outer race 18 and the required installation space increases, so another method is desired in order that these problems do not occur.
In addition, by using construction wherein the tangential force is supported by the outer peripheral surface of the outer race 18, there is a problem in that the deformation of the track groove portion of the outer race 18 becomes large as the stress at the portion on the inside surface side of the outer peripheral portion of the outer race 18 increases. In this case, as the deformation of the track groove increases, the resistance to the rolling of the rolling bodies (balls) increases. Moreover, due to this rolling resistance, there is a possibility that the transmission efficiency of the overall transmission will decrease.
Incidentally, a traction drive toroidal continuously variable transmission such as that of the second example and third example of conventional construction transmits power by using a special fluid (traction fluid) that transitions to a glass form under high pressure. Therefore, in a state of power transmission, it is necessary that a pressure force that is proportional to the transmitted torque be applied at the contact point between the disks 1a, 1b, 6 and power roller 8a. By applying this pressure force, the power roller 8a that is located between the disks 1a, 1b, and the trunnion 9a that supports that power roller 8a, receive a large force that acts as if to move them toward the outside. In order to support this force, a pair of yokes is placed above and below the trunnion 9a such that the tilt shafts 13 of the trunnion 9a are supported.
This pair of yokes regulates the movement toward the outside of the trunnion 9a. When there is no yoke, there is a possibility that the power roller 8a will shift from the correct position, causing the efficiency to drop and the life to decrease. Circular shaped attachment holes are formed in the central section in the width direction of each of the pair of yokes, where the inner peripheral surface of the attachment holes is a spherical concave surface, and posts having a spherical surface that are supported by a member attached to the casing of the toroidal continuously variable transmission fit inside the holes such that the yokes are supported by the spherical surfaced posts so that they freely pivot. However, the space between the trunnion 9a and the yokes is not regulated, and there is a high possibility that the trunnion 9a and yokes will come in contact with each other.
In this state, when the trunnion 9a rotates or tilts around the tilt shafts 13 (center axis of the tilt shafts 13), the shoulder section (base end section on the support beam section 15 side of both end sections 36) of the trunnion 9a and the yokes rub against each other, causing the friction resistance to increase. This increase in friction resistance hinders the tilt movement, and there is a possibility that operation will become unstable.
In order to handle this problem, a method of reducing the friction during tilting by forming protrusions on the yokes where the yokes come in contact with the trunnion 9a in order to reduce the contact surface area is disclosed in Japanese Patent Application Publication No. 1995(H07)-174201, for example. However, in the invention disclosed in this document, by forming protrusions on the yokes that come in contact with the trunnion, friction can be reduced, however, wear of the trunnion due to contact with the yoke cannot be prevented. In addition, forming protrusions on the yokes brings about an increase in cost.
Moreover, due to contact between the trunnion 9a and yokes, wear of the trunnion 9a occurs at the areas of contact between the trunnion 9a and the yokes. Therefore, in Japanese Patent Application Publication No. 2004-286053, a method is disclosed in which wear is prevented by tempering the area of the trunnion 9a that comes in contact with the yokes in order to increase the surface hardness. By tempering, it is possible to prevent wear of the trunnion, however, that does not mean that friction can be reduced. In addition, finishing of the trunnion is necessary, so an increase in cost is unavoidable.
In Japanese Patent Application Publication No. 2005-121045, a technique is proposed of preventing wear of the trunnion 9a by using construction wherein a plate is sandwiched and held between radial needle bearings 16 that are provided on the tilt shafts 13 of the trunnion 9a in order to function as tilt bearings, and the shoulder portion of the trunnion 9a, and bringing this plate in contact with the yokes. In this case, there is no increase in cost for processing the trunnion and yokes, however, similarly, this does not mean that friction can be reduced.
Furthermore, in Japanese Patent Application Publication No. 2001-323982, a method is disclosed whereby the both friction and wear are reduced by forming an arc shaped groove on one of the trunnion 9a and the yokes in the area of contact between them, which is formed around the center axis of the tilt shafts 13, and placing a plurality of balls in that groove so that there is rolling contact between the trunnion 9a and yokes. In this case, both friction and wear can be reduced, however, it is necessary to form an arc shaped groove on the trunnion, for example, and because that groove will be the surface over which the balls will pass, the surface of the groove must be processed, which increases the cost. In addition, there are also problems of workability in that balls may fall during assembly, and measures must be taken so that there is no shifting of position.
On the other hand, in a toroidal continuously variable transmission, when a high torque transmission capacity is needed, for example, in cases where it is installed in a vehicle having a large amount of exhaust, heat generated at the point of contact between the disks 1a, 1b, 6 and the power roller 8, 8a, or heat generated in the power roller bearing unit (thrust ball bearing 17) causes the breakdown or softening of residual austenite as the power roller 8, 8a becomes tempered, so there is a possibility of a decrease in durability, or a drop in the set value for μ (operation traction coefficient) as the temperature of the traction surface increases, or in other words, various problems occur such as an increase in pressure force between the input-side and output-side disks 1a, 1b, 6 and the power roller 8, 8a, and there is a further decrease in the power transmission efficiency.
Therefore, a method is proposed where by increasing the radius of the outer track groove (radius of curvature of the inner peripheral surface of the outer track groove) of the outer race with respect to the radius of the inner track groove (radius of curvature of the inner peripheral surface of the inner track groove) of the power roller 8, 8a, which functions as the inner race of the thrust ball bearing 17, a spin control state is formed in which there is pure rolling between the inner race and the rolling bodies, and there is spin slippage between the outer race and the rolling bodies (for example, refer to Japanese Patent Application Publication No. 2001-122200). In other words, through this inner race spin control, the heat generated on the outer race side of the thrust ball bearing 17 becomes large, and it is possible to suppress heat being generated on the inner race or power roller 8, 8a side, and by suppressing the heat generated by the power roller 8, 8a, the traction coefficient can be improved as well as the power transmission efficiency can be improved. However, when suppressing the heat generation by the power roller 8, 8a, which is the inner race, as the inner race spin control, there is a problem of damage to the outer race side. In addition, in the thrust ball bearing 17, the heat generated by the outer race side becomes large, so heat resistance and durability of the outer race 18 become a problem.
Moreover, the thickness of the outer race 18 is thin with respect to the power roller 8, 8a, so the outer race 18 deforms easily. Furthermore, in the surface hardening process, such as carburizing, carbonitriding, high-frequency tempering and the like, the depth of the surface hardened layer cannot be made deep. Therefore, in order to prevent damage to the outer race, a method is disclosed (for example, refer to Japanese Patent No. 3,733,992), where opposite to the example described above, as outer race spin control, the radius of the track groove of the inner race is made larger than the radius of the track groove of the outer race, making it possible to lower the temperature of the outer race. However, when trying to prevent damage to the outer race 18 as outer race spin control, the heat generated by the power roller 8, 8a becomes large, and thus there is a possibility of problems occurring such as a decrease in power transmission efficiency due to the rise in temperature of the power roller 8, 8a. 
On the other hand, a technique is disclosed (for example, refer to Japanese Patent No. 4,100,072) in which the durability of the rollers is maintained by increasing the heat resistance of the rollers based on the composition of materials used in the rollers of a full toroidal continuously variable transmission. However, this document discloses a composition of materials preferable for the rollers of a full toroidal continuously variable transmission and not a half toroidal type. The bending stress that acts on a half toroidal power roller 8, 8a is larger than that acting on a full toroidal power roller, so with the composition given in this document, cracking is a concern when manufacturing a power roller 8, 8a. 
Furthermore, a method is disclosed (for example, refer to Japanese Patent Application Publication No. H08-326862) that improves the rolling fatigue characteristics of a power roller 8, 8a through the composition of the materials used in a power roller 8, 8a of a toroidal continuously variable transmission, and by surface hardening. In this document, the composition of materials of a power roller 8, 8a employed is suitable to high-frequency tempering, however, neither the heat generation in the thrust ball bearing 17 nor the relationship of the material characteristics with the other construction have been considered. Therefore, depending on the construction of the thrust ball bearing 17 and the construction of the outer race 18, there is a possibility that the disclosed composition of materials is not the most suitable composition.